The present invention relates generally to process control networks and, more specifically, to a device and method for enhancing the capability of function block elements within a process control environment.
Process control networks, such as those used in chemical, petroleum or other processes, have generally included a centralized process controller communicatively coupled to one or more field devices which may be, for example, valve positioners, switches, sensors (such as temperature, pressure and flow rate sensors), etc. These field devices may perform physical control functions within the process (such as opening or closing a valve), may take measurements within the process for use in controlling the operation of the process or may perform any other desired function within the process. Process controllers have historically been connected to field devices via one or more analog signal lines or buses which may carry, for example, 4-20 mA (milliamp) signals to and from the field devices. Generally speaking, the process controller receives signals indicative of measurements made by one or more field devices and/or other information pertaining to the field devices, uses this information to implement a typically complex control routine and then generates control signals which are sent via the analog signal buses to the field devices to thereby control the operation of the process.
Recently, there has been a move within the process control industry to implement field-based digital control and communications within the process control environment. For example, the process control industry has developed a number of standard, open, digital or combined digital and analog communication protocols such as the HART(copyright), PROFIBUS(copyright), WORLDFIP(copyright), Device-Net(copyright) and CAN protocols. These digital communication protocols enable more field devices to be connected to a particular bus, support more and faster communications between the field devices and the controller and/or allow field devices to send more and different types of information, such as information pertaining to the status and configuration of the field device itself, to the process controller. Furthermore, these standard digital protocols enable field devices made by different manufacturers to be used together within the same process control network.
Also, there is now a move within the process control industry to decentralize process control and, thereby, simplify process controllers. Decentralized control is obtained by having field mounted process control devices, such as valve positioners, transmitters, etc., perform one or more process control functions using what are typically referred to as function blocks and by then communicating data across a bus structure for use by other process control devices (or function blocks) in performing other control functions. To implement these control functions, each process control device includes a microprocessor having the capability to implement one or more function blocks as well as the ability to communicate with other process control devices using a standard and open communication protocol. In this manner, field devices can be interconnected within a process control network to communicate with one another and to perform one or more process control functions forming a control loop without the intervention of a centralized process controller. The all-digital, two-wire bus protocol now being promulgated by the Fieldbus Foundation, known as the FOUNDATION(trademark) Fieldbus (hereinafter xe2x80x9cFieldbusxe2x80x9d) protocol is one open communication protocol that allows devices made by different manufacturers to interoperate and communicate with one another via a standard bus to effect decentralized control within a process.
As indicated above, some of these communication protocols, as well as some controller protocols used to implement or perform process control (such as proprietary controller protocols), use a basic building block typically referred to as a function block to implement specific parts of control routines or communication routines. For example, the Fieldbus protocol defines numerous function blocks that perform input functions (such as measuring the output of a sensor), control functions (such as implementing a proportional-integral (PI) or a proportional-integral-derivative (PID) control algorithm, a tuning algorithm, etc.), output functions (such as driving a valve positioner to open or close a valve) or other functions within a process control network. While these function blocks can be initially created and changed as desired to perform different functions within a process control environment, function blocks are generally static in nature once they are compiled and connected within a controller or communication environment. Upgrading an installed function block or enhancing the capability of an installed function block to perform additional or modified functions requires the operator to create a new function block with the upgraded capabilities and to recompile the new function block within the controller or communication environment, which can be time consuming and can require the process or part of the process to be taken off line. Furthermore, it is difficult to provide conditional capabilities for function blocks. That is, function blocks must typically be compiled within a process control or communication environment to include all of the capabilities that will ever be used within the function block, even if some of those capabilities are not needed for a particular purpose or are used only infrequently. One example of this is providing tuner capabilities within a function block that performs PI or PID control. Currently, if a manufacturer wants to provide tuner capabilities for a PI or PID control function block, the manufacturer must initially include such tuner capabilities in the PI or PID control function block before providing such a control block to a user. Thereafter, these tuner capabilities must be compiled and installed with the PI or PID control block whenever that control block is used, even if the user does not need or want the auto-tuning capabilities of the control block. In such an instance, the added functionality of the tuner merely takes up overhead and processing time in the installed routine.
A modifier function block that is adapted for use in process control routine includes a series of processor implementable instructions that conditionally enhance or modify the functionality of a base function block associated with the process control routine. The base function block includes a resettable branching point located within a set of instructions used to perform a process control related operation. The branching point of the base function block has a pointer which, when set to a first value, causes another of the set of instructions within the base function block to be executed (to thereby bypass the execution of the modifier function block), and which, when set to a second value, causes the series of instructions within the modifier function block to be executed (to thereby alter the functionality of the base function block). In this manner, the modifier function block may be conditionally called, depending on the state of the base function block, to change the operation of the base function block or to, alternatively, leave the functionality of the base function block unaltered.
The modifier function block may include an interface that defines the data associated with the base function block which is to be used by the modifier function block and may also include a return instruction that returns processor control to the set of instructions within the base function block when each of the series of instructions within the modifier function block has been executed.